A high-speed optical communication system has been popularized so as to realize large-capacity information communication. On a reception side of the optical communication system, a received optical signal is photocurrent-converted by a photodiode. A high-speed current signal which is outputted from the photodiode is converted into a voltage signal by a transimpedance amplifier (TIA) and at the same time, amplified to voltage amplitude adapted to analog-digital conversion of the next stage.
A differential system exhibiting high noise resistance is employed so as to voltage-convert/amplify a high-speed current signal. FIG. 1 is a configuration diagram of a differential amplifier circuit of related art (for example, refer to S. Mohan, et al., “Bandwidth Extension in CMOS with Optimized On-Chip Inductors”, IEEE Journal of Solid-State Circuits, Vol. 35, No. 3, pp. 346-355, March 2000). An output of a source (or emitter) set stage 112 is inputted into a source follower (or emitter follower) stage 111 and is fed back to an input of the source (or emitter) set stage 112 via a feedback resistor Rf1. In a similar manner, an output of a source (or emitter) set stage 122 is outputted to a source follower (or emitter follower) stage 121 and is fed back to an input of the source (or emitter) set stage 122 via a feedback resistor Rf2.
In this case, stability of feedback loops FBLP1 and FBLP2 depends on poles of the differential stages 112 and 122 and the source (emitter) follower stages 111 and 121. In order to avoid oscillation of the feedback loops FBLP1 and FBLP2 and stabilize an operation, a resistance value and a current/voltage value are changed and thus an arrangement of poles and gains is adjusted. However, if these parameters are changed, operation points of respective nodes are also changed. In the configuration of related art, it is difficult to adjust an arrangement of poles and gains without changing operation points.
A gain of an amplifier is obtained on the basis of a transfer function of a feedback circuit.
                              Z          T                =                                            v              o                                      i              i                                ≡                                    w              n              2                                                      s                2                            +                              2                ⁢                                                                  ⁢                ζ                ⁢                                                                  ⁢                                  w                  n                                ⁢                s                            +                              w                n                2                                                                        (        1        )            
Here, an attenuation coefficient is expressed as the following.
                    ζ        =                                            1              2                        ⁢                                                            T                  1                                +                                  T                  2                                                                                                  (                                          G                      +                      1                                        )                                    ⁢                                      T                    1                                    ⁢                                      T                    2                                                                                =                                    1              2                        ⁢                                                            f                  1                                +                                  f                  2                                                                                                  (                                          G                      +                      1                                        )                                    ⁢                                      f                    1                                    ⁢                                      f                    2                                                                                                          (        2        )            
Here, G denotes a gain of an amplifier.
                                          f            1                    =                                    1                              2                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                                  T                  1                                                      =                                          -                                                      p                    1                                                        2                    ⁢                                                                                  ⁢                    π                                                              =                              1                                  2                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                                      C                    1                                    ⁢                                      R                    1                                                                                      ⁢                                  ⁢                              f            2                    =                                    1                              2                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                                  T                  2                                                      =                                          -                                                      p                    2                                                        2                    ⁢                                                                                  ⁢                    π                                                              =                              1                                  2                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                                      C                    2                                    ⁢                                      R                    2                                                                                                          (        3        )            
p1 denotes a pole of a differential stage and p2 denotes a pole of an emitter follower.
When a resistance value is increased for gain adjustment, a frequency band becomes narrower. Further, when an electrostatic capacity of a capacitor is increased for phase compensation (stabilization of an operation), as well, a frequency band becomes narrower.
Stability of a feedback circuit may be estimated on the basis of a Q value. A Q value is expressed as Q=1/(2ζ). FIG. 2 is a graph illustrating a frequency property of a standardization transfer function corresponding to a Q value. When a Q value is increased, peaking appears. Large peaking represents instability of a feedback circuit. In order to planarize group delay characteristics without peaking, it is preferable to set a Q value to approximately 0.6.
Japanese Laid-open Patent Publication No. 10-117124 and Japanese Laid-open Patent Publication No. 2011-124711 are examples of related art.
It is desirable to provide an amplifier which is capable of adjusting a gain and realizing a stable operation (phase compensation) without changing an operation point and without narrowing a band, and an optical receiver which uses the amplifier.